1. Field of the Invention
The present invention relates to a cable network, and more particularly, to a method of controlling MACs between a cable modem termination system (CMTS) supporting variable length packet transmission and a cable modem (CM).
2. Discussion of the Related Art
Generally, a cable network is implemented by a packet data communication system supporting various services with very high speed.
A transmission path over the cable network is realized by a cable modem termination system (hereinafter abbreviated CMTS) and a cable modem (hereinafter abbreviated CM). Namely, CMTS and CM construct the cable network.
CMTS includes a network terminator for interfacing an external network (e.g. backbone network) distributed on an enormous area as well as a modulator/demodulator for an interface between upstream and downstream.
Streams of CMTS and CM are exchanged through a radio frequency (RF) interface.
CMs are connected to a plurality of customer premise equipments (CPEs) to realize a single local distribution network.
The cable network provides a customer with unidirectional and bi-directional services (e.g. cable broadcasting service, packet telephony service, video conferencing service, and services over Internet protocol) supported by the external backbone network
For instance, a structure of a cable network providing Internet protocol based services is shown in FIG. 1.
The cable network structure in FIG. 1 is based on DOCSIS (data over cable service interface specification).
Specifically, DOCSIS proposes a scheduling for securing quality of service (QoS) of VoIP (voice over Internet protocol) The scheduling is defined by a MAC (media access control) sub-layer as a sub-layer of a link layer among protocol layers of DOCSIS, and is an operation for allocating available resources effectively. Specifically, the scheduling is performed to share a plurality of service sessions for a single QoS. In aspect of a physical layer, the scheduling is performed to multiplex a plurality of service data with a single physical channel.
There is a UGS (unsolicited grant service) type or a UGS/AD (unsolicited grant service with activity detection) type as a scheduling type for supporting a real-time service flow generating variable-length packet data.
The UGS type occupies a bi-directional time slot regardless of an active state transmitting valid voice packets or an inactive state transmitting invalid voice packets.
On the other hand, the UGS/AD type occupies the bi-directional slots in an active state but occupies the time slots required for transmitting the valid voice packets only in an inactive state by monitoring periodically a count of time slots to transmit the valid voice packets.
The scheduling proposed by DOCSIS of the related art selects to use one of the two scheduling types, and alters to use the selected scheduling type to save the occupied bandwidth in case.
However, the alteration of the scheduling type according to the related art is applied to a plurality of the entire service sessions sharing the single physical channel. Namely, the UGS type is used for a plurality of the service sessions sharing the single physical channel. If allocation of the time slots are requested, as a new session is added while there is no time slot to allocate currently, the scheduling type of all the service sessions sharing the single physical channel is switched from the UGS type to the UGS/AD type to allocate the time slot, which is allocated for the transmission of the invalid voice packets, for the new session. Hence, the scheduling type (ex. UGS type) of a plurality of the service sessions sharing the single physical channel is collectively switched into the UGS/AD type. In other words, it is impossible to switch the scheduling type individually for the service sessions sharing the single physical channel.
After all, the switch of the scheduling type proposed for the purpose of avoiding waste of the occupied bandwidth has no effect occasionally.